1. Field of Invention
The present invention relates to an enhanced gas to liquids process for the production of heavy hydrocarbon products from carbonaceous feeds such as coal, biomass, municipal waste and/or light gaseous hydrocarbons such as natural gas, associated gas, coal seam gas, landfill gas, or biogas.
2. Related Art
Various processes are known for the conversion of carbonaceous feeds and/or light hydrocarbon containing gases into normally liquid products such as methanol, higher alcohols and hydrocarbon fuels and chemicals, particularly paraffinic hydrocarbons. Such processes are directed at the objective of adding value to the feedstock by making a transportable, more valuable product such as diesel fuel, jet fuel, or chemicals such as base oils, solvents, or drilling fluids.
The Fischer-Tropsch process can be used to convert such carbonaceous feeds and/or gaseous light hydrocarbon products into more valuable, easily transportable liquid hydrocarbon products and chemicals. The feedstock is first converted to synthesis gas comprising carbon monoxide and hydrogen. The synthesis gas is then converted to heavy hydrocarbon products over a Fischer-Tropsch catalyst. The heavy hydrocarbon products can be subjected to further workup or processing by hydroprocessing such as hydrocracking and/or hydroisomerization and distillation resulting in, for example, a high yield of high quality middle distillate products such as jet fuel or diesel fuel. The heavy hydrocarbon products can thereafter also be upgraded to specialty products such as solvents, drilling fluids, waxes or lube base oils due to the high purity of the Fischer-Tropsch products.
Processes that convert carbonaceous feeds and/or light hydrocarbons to heavier hydrocarbon products generally have at least three steps: 1) conversion of the feedstock to synthesis gas comprising carbon monoxide and hydrogen; 2) conversion of the synthesis gas to heavy hydrocarbons via a Fischer-Tropsch reaction; and 3) hydroprocessing the heavy hydrocarbon product to one or more finished hydrocarbon products.
The efficiency and effectiveness of the subject process depends not only on the effectiveness of the three steps, but also on how the steps are integrated. Efficient utilization of tail gas generated during the Fischer-Tropsch reaction reduces the amount of natural gas feed required. The present invention is directed to operate the Fischer-Tropsch reactors below the stoichiometric H2:CO ratio, resulting in a carbon rich tail gas which is partially purged as fuel gas. A hydrogen rich purge stream from hydroprocessing is added to the remaining tail gas so that the recycle stream is relatively high in hydrogen which results in improved operation of the syngas generator.
The present invention is directed to an efficient method of integration of the process steps comprising:
a process to convert carbonaceous feeds and/or light hydrocarbon gases into heavy hydrocarbon products which process comprises:                a) converting a feedstock of carbonaceous materials and/or light hydrocarbon gases into synthesis gas comprising hydrogen (H2) and carbon monoxide (CO);        b) separating a part of the hydrogen from the synthesis gas of step (a);        c) passing all or a portion of the synthesis gas after hydrogen separation over a cobalt based Fischer-Tropsch catalyst in a Fischer-Tropsch reactor resulting in production of heavy hydrocarbon products, water and a tail gas comprising mainly CO, CO2, H2, and C4− hydrocarbons and inert gases if present in the syngas feed;        d) using a portion of the H2 of step (b) for hydroprocessing the Fischer-Tropsch hydrocarbon products;        e) using a portion of the Fischer-Tropsch tail gas as fuel; and        f) mixing all or a part of the any remaining H2 rich stream from step (b) not used for hydroprocessing with all or a part of any remaining Fischer-Tropsch tail gas not used as fuel and recycling this mixed gas stream to step (a).        
U.S. Pat. Nos. 4,579,985, 4,587,008, and 4,628,133 each describe a process for the preparation of hydrocarbons wherein a light hydrocarbon stream and a recycle stream are reacted with steam in a steam methane reformer (“SMR”) to produce synthesis gas. The synthesis gas is reacted in a Fischer-Tropsch reaction to produce a mixed hydrocarbon stream including unreacted synthesis gas and light hydrocarbons. A portion of the hydrogen is removed from the tail gas and all or a portion of the gaseous stream of light hydrocarbons and unreacted synthesis gas after hydrogen separation is recycled to combine with the feed gas for feed to the SMR.
In the present invention, step b refers to the removal of hydrogen from the syngas generation in step a. It is advantageous to remove hydrogen immediately after syngas generation since this stream typically contains the highest partial pressure (concentration) and hence involves the lowest capital and energy cost for removal. The preferred amount of hydrogen to remove depends upon achieving the desired H2/CO ratio for the FT synthesis (step c) and to provide sufficient hydrogen for subsequent hydroprocessing of the FT liquids into final products (fuels, solvents, base oils, etc.). The appropriate amount of H2 for the syngas ratio adjustment depends on the configuration of the FT reactor system. Preferably the net amount of hydrogen in the Fischer-Tropsch system, including any hydrogen added back, is enough to assure that the H2:CO ratio in the Fischer-Tropsch tail gas will not drop below 0.5:1. A ratio below 0.5:1 may result in carbon build up on the catalyst and resulting loss of catalyst activity.
A portion of this hydrogen lean Fischer-Tropsch reactor tail gas is used as fuel gas. This tail gas stream contains CO2 and serves to purge CO2 from the system. Preferably, the excess hydrogen purged from the hydroprocessing step is not used as fuel so that a mixture of the remaining hydrogen and tail gas is recycled back to the syngas generation step. In a preferred embodiment, it is not necessary to remove CO2 from the recycle loop. Such CO2 removal adds capital and operating expense and is therefore undesirable.
U.S. Pat. No. 6,696,501 describes a method for conversion of natural gas to higher hydrocarbons comprising:                a) reacting natural gas with steam and oxygen in an ATR to produce synthesis gas;        b) passing the synthesis gas to a Fischer-Tropsch reactor where heavy hydrocarbon product, water and tail gas are produced;        c) separating the hydrocarbon product, water and tail gas;        d) reacting at least part of the tail gas stream with steam in a steam methane reformer (“SMR”); and        e) adding the reformed tail gas to the syngas stream before the Fischer-Tropsch reactor.        
The present invention does not require a separate reformer to efficiently recycle the tail gas.
U.S. Pat. No. 6,784,212 describes a process wherein synthesis product gases are separated in several steps and light gases are separated by pressure swing adsorption or are cryogenically separated such that CO and H2 are concentrated and recycled to the Fischer-Tropsch reactor, a portion of which may be concentrated in purified hydrogen. Optionally, the light gases are all recycled to the syngas production unit with no hydrogen separation. In the cryogenic separation option, CO2 is removed and vented.
The present invention does not use cryogenic or pressure swing adsorption technologies to separate tail gas components.
U.S. Pat. No. 7,250,450 describes a process wherein gaseous hydrocarbon feed is reacted with oxygen in a partial oxidation reactor and converted to synthesis gas. The synthesis gas is converted to hydrocarbon products in a Fischer-Tropsch reactor with a cobalt based catalyst. The hydrocarbon products are separated from the light components which are called a recycle stream, and CO2 is removed from the recycle stream before it is sent back to the partial oxidation unit.
The present invention does not use CO2 removal. It is an objective of the present invention to avoid CO2 removal as it is costly.
Accordingly, the present invention is directed to an enhanced process for the production of heavy hydrocarbon products from carbonaceous materials and/or a light hydrocarbon gas wherein a hydrogen depleted Fischer-Tropsch tail gas is partially consumed as fuel, therefore, purging CO2 from the system. The remaining tail gas is blended with a hydrogen purge stream and recycled back to syngas generation resulting in an enhanced process.
The present invention is also directed to an enhanced process resulting in increased production of heavy hydrocarbon products versus light hydrocarbon products.